Diesel engines rely on effective dispersion of fuel to
ensure efficient combustion. In cold weather regions,
maintaining the fluidity flow of fuels can be difficult.
Catalytic dewaxing is a selective hydrocracking process that
provides a valuable improvement to low temperature performance
of middle distillate feedstocks. It greatly improves
cloud point (CP) and cold-filter plugging point (CFPP)
properties of diesel fuels.

Background

At present, most US refineries are optimized for the
production of gasoline, i.e., fluid catalytic cracking (FCC)
units. With the growing interest in diesel-powered passenger
cars, existing refineries will not be able to serve that new
demand for clean diesel. Investments in new technologies to
produce high-performance transportation fuels will be
necessary. Fuel performance in diesel engines is directly
linked to fluidity characteristics in the engine. The highly
sophisticated injection technology relies on quick and
complete dispersion within the combustion chamber. At low
ambient temperatures, the cold-flow properties of typical
middle distillate cuts are not adequate.

Several options are typically applied to improve cold-flow
properties in diesel transportation fuels, including kerosine
blending, undercutting, use of additives (mainly at fuel
terminals) and catalytic dewaxing. Combined kerosine
blending/undercutting with the addition of cold-flow improvers
has some applicability, although it does not work in all cases.
As high-value kerosine is mainly used for jet fuel, blending
into lower-value diesel fuel is only acceptable if there is no
alternative outlet. Seasonal undercutting of middle-distillate
fractions will reduce total diesel yield as higher
boiling-point fractions end up in the low-value fuel oil (FO)
pool.

The application of versatile cold-flow improvement
additives, typically done at product terminals or blending
sections, is very efficient to tailor flow properties like
viscosity index (VI) or pour point (PP). However, the impact on
cold-flow filter plugging point is limited, and the impact on
CP may even be negative in some cases.

A selective hydrocracking catalyst has proven to be a robust
approach for catalytic dewaxinga process that can be used
to address all aspects of cold-flow
performance.a

Middle-distillate properties

For diesel fuel, middle distillates have boiling-point
curves in the range of 150°C (300°F) to 400°C
(750°F). In addition to environmental specifications
regarding sulfur, nitrogen and aromatics impurities, combustion
behavior (cetane number and heating value), viscosity and flow
behavior specifications are important performance factors for
diesel fuels. The top four globally standardized properties
are:

VI

PP

CP

CFPP.

Whereas VI and PP primarily describe the quality of the
middle-distillate fluidic behavior and its ability to be
transported from tank to engine, CP and PP describe the ability
to filter and disperse the fuel at lower temperatures. The VI
is calculated by the kinematic viscosity at 40°C
(100°F) and 60°C (140°F). At higher VIs, the change
of kinematic viscosity with temperature is lower. The PP is the
lowest temperature at which a liquid will pour or flow under
prescribed conditions. It is an approximate indication of the
lowest temperature at which the liquid can still be pumped.

The CP is the temperature at which small crystals occur
(turbidity) in defined measurement equipment. The CFPP is the
temperature at which a filter starts to plug in a defined
filtration set-up. If seasonal cold-flow specifications are not
met, an unexpected cold snap can lead to equipment damage, as
shown in Fig. 1.

The cracking of middle distillate to select paraffinic and
isoparaffinic molecules and their melting points is summarized
in Fig. 2. As the melting point of a
particular hydrocarbon molecule in the
middle-distillate fraction is strongly linked to cold-flow
properties, middle distillates with a high content of
isoparaffins have some advantages. Therefore, middle
distillates with more paraffinic hydrocarbons but poor
cold-flow properties can be converted into middle distillates
with good cold-flow properties by increasing the
isoparaffin-to-paraffin ratio.

Two types of catalytic conversion are possible: dewaxing by
isomerization, and dewaxing by selective cracking. A catalyst
system can selectively crack paraffinic hydrocarbons of
middle-distillate feedstocks.a The cracking
function in this novel catalyst is performed by a solid-acid
ingredient based on a medium pore-size zeolite that
shape-selectively differentiates between branched isoparaffins
and linear normal-paraffins. As shown in Fig.
3, only unbranched normal paraffins (n-paraffins) can
enter the pores and be converted into smaller molecules via
cracking. The catalyst includes a zeolite with a unique acidity
profile that provides outstanding robustness and flexibility
for use with a variety of feedstocks.a In addition,
a second catalytic-base-metal function allows fast hydrogen
transfer for efficient product release and coke
prevention.

Dewaxing service

The catalyst has been commercially available for nearly 20
years. It can be used as a stand-alone solution, or within an
existing middle-distillate hydrotreater or ultra-low-sulfur
diesel unit, as shown in Fig. 4. Middle
distillates with a wide variety of cut points can be processed.
As basic nitrogen has a particular influence on total catalyst
activity, the placement of a small bed of cobalt
(Co)-molybdenum (Mo) or nickel (Ni)-Mo hydrotreating catalysts
in front can be helpful, particularly for a stand-alone
operation.b

Fig.
4. Dewaxing by selective cracking with
hydrocracking catalyst in a
stand-alone unit (a), or within an existing
middle-distillate hydrotreating unit (b).

Using selective-hydrocracking catalysts within an existing
middle-distillate hydrotreating unit is a very common practice.
This catalyst can be used with nearly all types of feedstocks, whether straight-run or
converted (such as cracker or visbreaker or coker gasoil) refinery product streams. Its
properties are tailored to fit to all hydrotreating catalysts
commercially available. Selective cracking is an endothermic
process; therefore, the placement of a hydrocracking catalyst
bed between two hydrotreating catalyst beds in a
hydrodesulfurization (HDS) reactor allows optimum heat integration and very favorable
product qualities for cold flow and color. Moreover, commercial
experience confirms the control of dewaxing activity according
to seasonal demand even without quenching capabilities. In some
cases, a two-reactor solution with bypass lines, as represented
in Fig. 5, is the most favorable.

The use of selective hydrocracking catalysts can moderately
reduce diesel production and increase hydrogen consumption,
depending on operational severity and cold-flow improvement
requirements. However, many selective hydrocracking
installations circumvent diesel yield loss by applying feedstock components with higher
final boiling point (higher cut point). This action allows the
conversion of a portion of non-blendable intermediates into
higher-value diesel components to compensate for reduced diesel
production.

An additional feature of the selective-hydrocracking
catalyst system is the reduced gas-make. This is very important
for existing hydrotreating units, as the formation of light
hydrocarbons has no big impact on recycle gas density.
Therefore, it does not interfere with the recycle-gas
compressor operation. Finally, using a selective dewaxing
catalyst as a drop-in replacement does not require cost
intensive revamp or exchange of recycle compressors. Only
minimal modifications of product stabilizers may be necessary
to handle higher naphtha volumes in rare cases.
HP

ACKNOWLEDGMENT

The article is a revised and updated version from an earlier
presentation at the American Fuel and Petrochemical Manufacturers (AFPM)
Annual Meeting, March 1719, 2013, at San Antonio,
Texas.

NOTES

a HYDEX-G is used for selective hydrocracking of
long-chain n-paraffins to improve the cold-flow properties of
middle distillates. Its most common application is for
sulfur-containing diesel streams in combination with HDS
catalysts in an integrated system. It is a registered product
of Clariant.b HDMax is a hydrotreating catalyst series
developed primarily for severe hydrotreating operation of waxes
and lube oil stocks. It is a registered product of
Clariant.

The authorsDr. Rainer Albert Rakoczy is the
global product manager for zeolite-based fuel upgrading
and fuel production catalysts with Clariant. He started
with Süd-Chemie in 2005 and headed the
solid-catalyst research department. Dr. Rakoczy studied
chemistry at the University of Stuttgart and worked
also in the field of PCB production (IBM and
Hewlett-Packard) and microprocess engineering (FZ
Karlsruhe). Dr. Rakoczy has a deep background in the
field of zeolites. He is an elected member of the
Zeolite Group board of the German ProcessNet
Association (DECHEMA).Dr. Paige Marie Morse is the global
marketing manager for the catalysts business of
Clariant; she is based in Munich, Germany. Previously,
she held technical and business development roles at
Dow and Shell in the US. Dr. Morse holds a PhD in
chemistry from the University of Illinois.

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